Subsidence due to peat compaction may have important implications in alluvial floodplains, because it leads to wetland loss, land inundation, and damage to buildings and infrastructure. Furthermore, it potentially influences spatial and temporal river sedimentation patterns, and hence the evolution of alluvial floodplains. As the vast majority of peatlands occur in cold temperate regions, alluvial plains situated in such areas are most susceptible to these implications. To determine which locations within alluvial floodplains are most vulnerable to high amounts of compaction-induced subsidence, it is necessary to quantify compaction and to identify which factors influence the amount and rate of peat compaction. For this, we carried out field work in the Cumberland Marshes (east-central Saskatchewan, Canada) guided by the following two questions: (1) how much peat compaction has occurred over the past decades to centuries, and (2) which factors control the amount and rate of peat compaction at the same timescale. To address these questions, we used methods involving construction of stratigraphic cross sections, organic-matter content and bulk-density measurements, and radiocarbon dating. A new sampling device was devised to sample uncompacted peat in the field. Results show that peat layers have compacted up to 43% within a few centuries, with compaction rates of up to 6.08 mm/yr. The dominant factors influencing peat compaction are: (1) organic-matter content, (2) stress imposed on a peat layer, and, to a lesser extent, (3) plant species composition. In an alluvial setting, crevasse splays and natural levees are sites that are most susceptible to high amounts of peat compaction at short timescales (100–102 years). Sheet-like splay deposits initially compact underlying peat uniformly, whereas differential compaction commonly occurs beneath natural levees due to lateral variations in sediment thickness. Subsidence due to peat compaction creates additional accommodation space and hence locally enhances floodplain sedimentation rates.